One of the most common types of hearing loss is sensorineural deafness which is caused by the loss of hair cells, sensory cells in the cochlea that are responsible for transduction of sound into an electrical signal. The human inner ear contains only about 15,000 hair cells per cochlea at birth, and, although these cells can be lost as a result of various genetic or environmental factors, the lost or damaged cells cannot be replaced. However, overexpression of the transcription factor, Atoh1, can induce the differentiation of hair cells from epithelial cells in the sensory organ of the cochlea, the organ of Corti ((Zheng and Gao, Nat Neurosci 2000; 3:580-586; Kawamoto et al., J Neurosci 2003; 23:4395-4400; Gubbels et al., Nature 2008; 455:537-541). Atoh-1 expression also plays a role in driving other cells, e.g., intestinal cells, into a differentiated state (Aragaki et al., Biochem. Biophys. Res. Comm. 2008 April; 368(4):923-929), and overexpression of Atoh-1 reduces proliferation of colon cancer cells (Leow et al., Ann N Y Acad Sci. 2005 November; 1059:174-83).